DC SQUID

A DC SQUID is just two Josephson junctions wired in a small superconducting loop. On a quantum chip its job is usually not to sense fields but to act as one tunable junction: by threading magnetic flux through the loop, you can dial its effective Josephson inductance up and down. Since a qubit's frequency is set by that inductance, this is the knob that lets you tune a qubit in place, or turn the coupling between two qubits on and off.

It works through interference. The two junctions sit on either side of the loop, and the magnetic flux passing through the loop shifts the relative quantum phase of the two paths. When the paths add in phase the loop behaves like a strong junction with a large critical current; as you add half a flux quantum (Phi_0 = h/2e) they fight, and the effective critical current shrinks toward zero. The pattern repeats every Phi_0, so a tiny coil or a current-carrying wire near the loop sweeps the qubit's frequency over a wide range. The same loop, biased near maximum sensitivity, makes one of the most precise magnetometers ever built.

The catch is that tunability and noise come together. Because the frequency now depends on magnetic flux, every stray magnetic fluctuation, from trapped vortices to current ripple in the bias line, jitters the qubit and shortens its coherence. Designers fight this by parking qubits at a flux 'sweet spot' where the frequency is flat to first order, by shielding and filtering, and by using asymmetric junctions. It is a real engineering trade, not a solved problem: you buy controllability with added flux-noise sensitivity.